1. Field of the Invention
The present invention relates in general to the detection of signal disturbances, and, in particular, to the detection, in partial response read channels, of signal disturbances such as those caused by thermal asperities.
2. Description of the Related Art
Data transmission systems are susceptible to noise from various sources. One such source of noise is a thermal asperity (TA), which can occur in the reading of data from magnetically recorded media, such as hard disk drives. A thermal asperity occurs when a magneto-resistive (MR) read head, which typically flies at a high speed and low height or distance over the surface of the hard disk drive platter, collides with a particle on the surface of the platter, with a bump or similar imperfection on the platter, or with the platter itself. The collision causes a spike in the temperature of the magneto-resistive read head, which therefore temporarily increases the resistance of the read element in the magneto-resistive read head, which in turn can cause erroneous data readings for the period corresponding to the temperature spike. The amplitude of a typical TA can exceed the value of a normal read signal by a factor of more than 10. Unless appropriately handled, a TA can substantially degrade system performance during its occurrence. Read errors due to a TA can last for several hundred bits, or longer.
FIG. 1 shows a typical thermal asperity disturbance, labeled as “Raw TA,” which shows a signal with a rapid rise and a slow decay. Some basic techniques known in the art to combat TAs include analog TA recovery, channel detection, coding, and Reed-Solomon erasure decoding, each of which involves the detection of a TA followed by an attempt to correct for the detected TA by adjusting some aspect of the processing of the signal corresponding to the duration of the TA.
For example, temporarily shortening the time constant τ of an AC coupler (ACC) in a read channel detector can substantially shorten the period of the TA, as can be seen in FIG. 1 by comparing the curve labeled “Raw TA” with the curve labeled “TA after ACC.” However, the TA must first be detected before corrective measures can be taken.
FIG. 2 shows a typical prior-art read channel detector 200 for detecting and correcting for TAs. TA detector 200 operates based on the difference in frequency between a TA and the normal read-back signal. A TA is a low-frequency disturbance, while normal read-back signals are high frequency. Therefore, one method to detect a TA is to use a low-pass filter. Since the high-frequency normal read-back signal is filtered out, the TA signal that remains can be detected by thresholding the amplitude of the surviving signal.
In particular, as shown in FIG. 2, signal 201a from the read head goes into pre-amplifier 201. The output of the pre-amplifier during a typical TA is shown in FIG. 1 as “TA after pre-amp.” Output 202a of the pre-amplifier goes into AC coupler (ACC) 202. An AC coupler, as is well known in the art, acts as a high-pass filter. ACC 202 has a variable time constant τ. Output 203a of the ACC goes into variable gain amplifier (VGA) 203, which smoothes the signal amplitude. Output 204a of VGA 203 forms an input for three elements of prior-art read-channel detector 200. Output 204a goes into continuous time filter 204, which is part of the normal signal-processing path not associated with the detection of TAs. Output 204a also goes into low-pass filter 205. Output 206a of low-pass filter 205 is compared by comparator 206 with a specified threshold value 206b to detect the rising edge of a TA. If comparator 206 determines that output value 206a of low-pass filter 205 is greater than threshold value 206b, then output 207b of comparator 206 indicates this to control logic 207, which generates control signal 202b to reduce the time constant τ of ACC 202, and thus changing the frequencies filtered by ACC 202. FIG. 1 shows output 203a of the ACC during a typical TA, after its time constant τ has been reduced, as “TA after ACC.”
Zero-crossing detector 208 detects the transition of the value of output 204a from positive to negative, as exemplified in the curve “TA after ACC” in FIG. 1, and thus confirms the occurrence of a TA. Output 207a of zero-crossing detector 208 goes into control logic 207. Based on inputs 207a and 207b, control logic 207 determines whether a TA has occurred, and, if so, outputs a signal 207c indicating that a TA has occurred. After the control logic has determined that the TA has passed, the time constant τ of ACC 202 is returned to its normal value. Thermal asperities and the prior-art method for their detection and correction are further described in “CMOS Circuits for Thermal Asperity Detection and Recovery in Disk-Drive Read Channels,” by A. Lee et al., IEEE International Midwest Symp. on Circuits and Systems, Tulsa, pp. III-364 to III-367, August 2002, incorporated herein by reference.
The prior-art systems for detecting TAs are effective for relatively large TAs, with amplitudes of about two or more times the amplitudes of normal read-back signals. However, the prior-art systems do not reliably detect TAs of relatively small amplitudes, such as those of about the same amplitude as the amplitude of a normal read-back signal. Such relatively small TAs are more common in perpendicularly recorded magnetic media than in longitudinally recorded magnetic media. Furthermore, relatively small TAs typically are not problematic in longitudinally recorded magnetic media because the read channels of such media use DC-free partial-response targets and the relatively small TAs can be effectively eliminated by the inherent equalization.
Perpendicularly recorded magnetic media allow for greater recording densities and their use is likely to overtake that of the easier-to-implement longitudinally recorded magnetic media. However, in perpendicular-recording read channels, DC-full or DC-partial targets are used, and small TAs cannot be effectively eliminated by equalization and therefore may cause long burst errors.
In addition, baseline wander, which can be caused by high-pass filtering of the low-frequency signals abundant in perpendicular recording, makes detection of relatively small TAs difficult. Furthermore, if the threshold for TA detection, as shown in FIG. 2 as signal 206b, is set too low, then frequent mis-detection of TAs (i.e., declaring something to be a TA which is not a TA) can occur. On the other hand, if the threshold is not low enough, then some small TAs will go uncorrected. In other words, it is difficult to differentiate a small TA from baseline wander. If small TAs are undetected and not handled appropriately, then the resultant error propagation in the read channel would be problematic for iterative decoding systems in next-generation devices.